A turbomachine is for example a steam turbine that has a stator and a rotor which is surrounded by the stator. The rotor has a shaft and a plurality of rotating blades which are mounted on the shaft in a group as rotating blade stages. The stator has a housing on which a plurality of guide vanes are provided, grouped together as guide vane stages, the guide vane stages being arranged correspondingly to the rotating blade stages.
During operation of the steam turbine steam is expanded over the rotating blade stages and the guide vane stages, thereby rotationally driving the rotor. This causes the rotor to be moved relatively with respect to the stator, with the result that the rotor can rub against the stator. In order to prevent the rubbing of the rotor on the stator as far as possible, an annular gap with respect to the stator is provided around the circumference of the rotor.
During operation of the steam turbine the stator and rotor are exposed to thermal stresses and the rotor in particular is exposed to mechanical stresses due to rotordynamic effects and steam forces. These stresses can lead to the annular gap being bridged during operation of the steam turbine due to deformation of the rotor and/or stator, with the result that the rotor rubs against the stator at least one point.
Generally the pressure of the steam in the steam turbine is reduced along its longitudinal direction, the pressure conditions propagating accordingly into the annular gap. As a result corresponding pressure gradients are produced in the annular gap, in particular in the longitudinal direction of the steam turbine, leading to a leakage flow of steam. The leakage flow causes a decrease in the internal efficiency of the steam turbine. In order to counteract this the height of the annular gap is chosen such that on the one hand the annular gap has the necessary clearance for reliable operation of the steam turbine and on the other hand the leakage flow through the annular gap is small.
Conventionally, a labyrinth seal is installed in the annular gap in order to reduce the leakage flow; said seal can be embodied for example as a see-through labyrinth, full labyrinth, stage labyrinth or as a comb-groove labyrinth.
FIG. 4 shows a conventional comb-groove labyrinth seal 101. An annular gap 104 having a gap height 105 is embodied between a rotor 102 and a stator 103. Arranged in the annular gap 104, the comb-groove labyrinth seal 101 has five sealing strips 106 disposed one after another in series, each being manufactured from an annular plate. The sealing strips 106 are rolled into the surface of the rotor 102 in such a way that the sealing strips 106 bridge the gap height 105 except for an outer radial clearance 109. The surface of the stator 103 is provided with grooves 107 and webs 108 located therebetween, with one of the grooves 107 and one of the webs 108 being arranged in alternation in each case facing one of the sealing strips 106.
If the comb-groove labyrinth seal 101 is provided in the steam turbine, the outer radial clearance 109 is dimensioned such that during operation of the steam turbine the outer radial clearance 109 is as good as never bridged by the sealing strips 106. A bridging of the outer radial clearance 109 can be caused for example by the rotor 102 being placed into radial oscillation due to rotordynamic effects during operation of the steam turbine. Furthermore the rotor 102 and the stator 103 can have a different thermal expansion due to different thermal stresses, with the result that the gap height 105 can vary during operation of the steam turbine.
Should at least one of the sealing strips 106 nonetheless touch the surface of the stator 103 during operation of the steam turbine, then said sealing strip 106 inscribes itself into the surface of the stator 103 with consequent material abrasion. At the same time the sealing strip 106 heats up due to friction, as a result of which a change in the material structure can occur in the sealing strip 106. This can lead to the strength of the sealing strip 106 being impaired.
The leakage rate of the comb-groove labyrinth seal 101 is essentially predetermined by the outer radial clearance 109, since this defines the effective cross-section of the comb-groove labyrinth seal 101. The smaller the outer radial clearance 109 of the comb-groove labyrinth seal 101, the higher is the leakage rate of the comb-groove labyrinth seal 101. Conversely, the larger the outer radial clearance 109 of the comb-groove labyrinth seal 101, the higher is the operating reliability against rubbing of the comb-groove labyrinth seal 101. Accordingly the comb-groove labyrinth seal 101 is conventionally provided with the outer radial clearance 109 of the kind in which the comb-groove labyrinth seal 101 has sufficient operating reliability, the resulting leakage rate of the comb-groove labyrinth seal 101 being caused by the outer radial clearance 109.